This invention relates to a specific gravity detecting device for a lead-acid battery and, more particularly, to the construction of an electrode-type specific gravity measuring unit and a method for stabilizing the potential of electrodes used for the unit.
In a lead-acid battery, the specific gravity of the electrolyte is raised and lowered as the battery is charged and discharged, respectively. Accordingly, the specific gravity of the electrolyte is indicative of how much the battery is charged or discharged. Various types of specific gravity sensors have been developed which provide an output that changes according to changes in the specific gravity of the electrolyte. A relatively simple one among these is the electrode-type sensor. In one version of the specific gravity sensor, the difference in potential between a reference electrode immersed in dilute sulfuric acid of known specific gravity and an electrode immersed in the electrolyte of the battery provides an indication of the specific gravity of the electrolyte of the battery. However, this type of specific gravity sensor is disadvantageous in that it is difficult to maintain the specific gravity of the electrolyte in the reference electrode constant and to prevent the reference electrode from deteriorating due to a self-discharge phenomenon. In another version of the specific gravity sensor of the electrode type, the difference in potential between two electrodes having different potential vs. specific gravity characteristics is employed as an indication of the specific gravity of the electrolyte. A typical example of such a sensor employs electrodes of lead and lead dioxide. However, if the lead or lead dioxide electrode is made of a nonporous material, lead sulfate tends to form on the surface thereof in a short time, thus preventing accurate measurement. In yet another electrode construction, a collector lattice is filled with porous lead or lead dioxide. This construction too is disadvantageous in that the lead dioxide electrode undergoes the elution of antimony, which then attaches to the lead electrode, thus undesirably changing the potential of the electrode. This construction is also disadvantageous in that there is a strong tendency for the lead dioxide electrode to undergo a vigorous self-discharge, resulting in changes in the volume thereof, sometimes causing the porous lead dioxide to become detached from the collector lattice.